1. Field
Embodiments of the present invention relate to a power supply control circuit connected to an alternating current power source via an input circuit having a capacitor, a power source cut-off detection method, and, particularly, a power supply control circuit and power source cut-off detection method such that a discharge of the capacitor is carried out when cutting off the alternating current power source.
2. Description of the Related Art
A power supply control circuit has heretofore been used as a common switching power supply in order to switch a full-wave rectified alternating current input voltage, generate a predetermined direct current voltage, and supply it to an external load. A circuit including a capacitor is normally used as an input filter used in a switching power supply for the purpose of noise removal. In order to safely dispose of residual electric charge accumulated in this kind of noise filter capacitor, a heretofore known arrangement is adopted such that a discharging resistor is connected in parallel to a capacitor, and when a plug is disconnected from AC lines, the residual electric charge of the capacitor is discharged via the resistor. With this method, as the discharging resistor is constantly connected to an alternating current input power source, a power loss occurs in the discharging resistor, and power consumption of the switching power supply increases, causing a decrease in power conversion efficiency.
FIG. 10 is a circuit diagram showing a configuration of a heretofore known switching power supply.
A rectifier circuit 103 is connected to AC lines 101 of a commercial power source via a filter circuit 102. The filter circuit 102 is configured of capacitors Cx1 and Cx2 and an inductor LF. A discharging resistor Rx which short-circuits opposing poles of the AC lines 101 is provided on the input side of the filter circuit 102.
Herein, the switching power supply is configured of a capacitor 104 and transformer 105 connected to the output side of the rectifier circuit 103, a switching element 106 connected in series to a primary winding 105P of the transformer 105, and a rectifying smoothing circuit 107 formed of a diode D and smoothing capacitor C connected to a secondary winding 105S of the transformer 105. Further, a turning on/off of the switching element 106 is controlled by an unshown PWM control circuit, thereby supplying a predetermined direct current voltage to a load (not shown) connected between direct current terminals 108a and 108b. 
In this way, the switching power supply is configured so that the filter circuit 102 is generally used as an input section, thus preventing noise being emitted to the AC lines 101. However, although the switching power supply is disconnected from the AC lines 101, electric charge remains in the capacitors Cx1 and Cx2 of the filter circuit 102, and there is a risk of getting an electric shock from the voltage of the electric charge. Therefore, a configuration is adopted such that a discharging resistor Rx is inserted between opposing poles of the AC lines 101, thus discharging the capacitors Cx1 and Cx2 when the switching power supply is disconnected from the AC lines 101.
The problem here is in that, as the discharging resistor Rx is constantly connected to the AC lines 101, power conversion efficiency decreases.
An electronic control device of an air conditioning system is disclosed in the below-cited related art patent literature document 1 (PTL 1). FIG. 11 is a circuit diagram showing the electronic control device configuring a heretofore known switching power supply disclosed in PTL 1.
The switching power supply of FIG. 11 includes a discharging resistor 202 between opposing poles of an alternating current input power source supplied from a plug 201, a resistor element 204 with resistance lower than that of the discharging resistor 202, to which a power failure detection element 203 which detects that an alternating current power source is being supplied is connected in series, and which is connected in parallel to these elements 202 and 203, and a switch 206 which is turned on/off by a controller 205 which operates by receiving a detection signal from the power failure detection element 203.
In the electronic control device 207 of PTL1, the resistor element 204 with a resistance value smaller than that of the discharging resistor 202 and the switch 206 are connected in series, it is detected that the plug 201 has been disconnected, and the switch 206 is turned on. Because of this, the resistor element 204 is connected only when the plug 201 is disconnected from the AC lines, meaning that it is possible to more quickly reduce a residual voltage between the opposing alternating current poles. The plug 201 supplies the alternating current power source to the electronic control device 207 via a connector 208, and the alternating current power source is supplied as the power source of the controller 205 through a noise filter 209 and a capacitor 210, and furthermore, a rectifying element 211 and a capacitor 212.
Herein, when the plug 201 is connected to the AC lines, no loss occurs as the discharging resistor 204 is disconnected from the circuit. As the discharging resistor 204 is connected when the plug 201 is disconnected, the capacitor 210 is discharged.
A problem of the switching power supply of PTL 1 is in that the whole structure becomes large because the switch 206 is used. Also, a particularly important problem includes a point of being short of specifics as no method of detecting that the plug 201 has been disconnected from the AC lines is disclosed.
Next, related art patent literature document 2 (PTL 2) cited below shows another switching power supply. FIG. 12 is a circuit diagram showing a configuration example of a heretofore known switching power supply disclosed in PTL 2.
Herein, a switching power supply arranged in such a way that a direct current voltage wherein an input alternating current voltage is rectified by a rectifier circuit 301 is switched by a main switching element Q, and control means 302 controls the switching in accordance with output voltage information feed back from the secondary side of a transformer N, thereby stabilizing an output voltage at a desired value, is characterized by including a plurality of series resistors R11 and R12 interposed on the input side of the rectifier circuit 301, and a resistor R2 which supplies a starting current from the connection point of the series resistors R11 and R12 to the control means 302.
In the switching power supply, a discharging resistor and control circuit starting resistor which have heretofore been individually provided are also used as the starting resistors R2, R11, and R12. Because of this, with the switching power supply of PTL 2, it is possible to reduce losses which have occurred individually.
However, with the switching power supply of FIG. 12, it is possible to reduce losses which have occurred individually, but it is not possible to eliminate a power loss completely.
Next, related art patent literature document 3 (PTL 3) cited below shows a direct current power supply arranged in such a way as to discharge a filter circuit capacitor. FIG. 13 is a circuit diagram showing a configuration example of a heretofore known switching power supply disclosed in PTL 3.
The direct current power supply includes an across-the-line capacitor Cy5 configuring a lowpass filter connected to both poles of an alternating current power source.
The direct current power supply includes first capacitors Cy1 and Cy2 connected to the respective poles of the alternating current power source, first diodes Dy3 and Dy5 whose anodes are connected to the other ends of the first capacitors Cy1 and Cy2, and second diodes Dy2 and Dy4 whose cathodes are connected to the anodes of the first diodes Dy3 and Dy5 respectively, one for each pole of the alternating current power source.
Also, the direct current power supply, as well as including an alternating current voltage rectifying bridge diode 401 with each pole of the alternating current power source as an input side, has a second capacitor Cy3 of which one end is connected to both of the cathodes of the two first diodes Dy3 and Dy5, and the other end is connected to the anodes of the two second diodes Dy2 and Dy4 and the negative pole of the alternating current voltage rectifying bridge diode 401. The direct current power supply further includes a first resistor Ry1 connected in parallel to both ends of the second capacitor Cy3, a first transistor Q3 into the gate of which is input a voltage across the second capacitor Cy3 and first resistor Ry1, and a second transistor Q4 into the gate of which is input the drain voltage of the first transistor Q3. The direct current power supply also includes a comparator 402 acting as an input voltage detector and a photo coupler 403 acting as a detector which emits a signal indicating that an external device driven by the direct current power supply is in operation.
Furthermore, in a condition in which the alternating current power source is connected, the first transistor Q3 is turned on, while the second transistor Q4 connected in cascade to the first transistor Q3 is turned off, causing no current to flow to a drain side resistor Ry4 or source side resistor Ry3 of the second transistor Q4. Also, when the alternating current power source takes on a disconnected state, the first transistor Q3 is substantially instantly turned off, and the second transistor Q4 is turned on, thus discharging the charging voltage of the across-the-line capacitor Cy5 within a predetermined time period through the drain side resistor Ry4 and source side resistor Ry3 of the second transistor Q4.
While an AC input is being supplied, the second capacitor Cy3 is constantly charged in a path from the first capacitor Cy1 through the first diode Dy3 to the second capacitor Cy3 or from the first capacitor Cy2 through the first diode Dy5 to the second capacitor Cy3. Because of this, the gate voltage of the first transistor Q3 becomes H (high, that is, the zener voltage of a zener diode Dy6), and the first transistor Q3 is maintained in an on-state. At this time, the gate voltage of the second transistor Q4 becomes L (low), and the second transistor Q4 is turned off.
Next, when the supply of the AC input is stopped, the second capacitor Cy3 is not charged, the gate voltage of the first transistor Q3 lowers, and the first transistor Q3 is turned off. Then, the gate of the second transistor Q4 is pulled up by the second resistor Ry2, and the second transistor Q4 is turned on. As the fourth resistor Ry4 connected to the second transistor Q4 is set to a low resistance, the across-the-line capacitor Cy5 is discharged for a short time.